For people who are starting the game and looking at professions for the very first time, some of the professions are fairly obvious in how they should be combined. If you're going to take leatherworking, you'll likely also be taking skinning so that you have some leather to work with. Mining is matched up with blacksmithing, jewelcrafting, and engineering. You've got alchemy and inscription intended to be with herbalism. Lastly, you've got tailoring and enchanting.
All you really need for enchanting is a profession that makes uncommon quality or better equipment. Enchanting doesn't really have to be matched with tailoring, but all of the other crafting professions need a gathering skill, and enchanting works better when you have another profession to disenchant from. The gathering skill for tailoring is just killing humanoids and undead over and over again, and thus it can be matched with any gathering profession just to fill a slot.
If you're anything other than a warlock, priest or mage, then taking tailoring is fairly unintuitive if you want enchanting. You could just combine it with a gathering profession like mining or skinning, but that means you need to obtain your enchant materials via the auction house, get really lucky via the dungeon finder or use alts with other professions.
What the game needs is an alternative, self-contained profession that could be matched with either. Could it be the woodworking profession idea that they announced had pretty much been scrapped during this past year's BlizzCon? How about merging leatherworking and skinning into a single profession? Or do you take a page out of Final Fantasy XI's book and go with bonecrafting (we just finished an expansion heavy on the bone armor theme)? What's your idea for a single slot crafting profession that would be useful to something other than cloth-wearers?